1. Field of the Invention
This invention relates to an image reading apparatus, commonly referred to as a "scanner" and, more particularly, to an image reading apparatus for reading 35 mm film and similar image recording mediums.
2. Description of the Related Art
A conventional image reading apparatus used to read an image on original 35 mm film is illustrated in FIG. 1.
A film original 3 is held by carriage 2 and a three-color separation filter 17 is arranged in front of carriage 2. Separation filter 17 is, for example, a filter for the primary colors of R (red), G (green), and B (blue). A white light source 16 is arranged in front of separation filter 17.
Mechanical changeover of separation filter 17 is conducted by filter drive circuit 18, which is controlled by CPU 13 via I/O 8. White light source 16 illuminates film original 3 through separation filter 17 and the transmitted light conducts image formation in CCD line sensor 4. Stepping motor 7 allows carriage 2 to move in a direction parallel with CCD line sensor 4. The formation image of CCD line sensor 4 is formed in a parallel manner (subscanning) by the parallel movement of carriage 2. CCD line sensor 4 is arranged so that its scanning direction is orthogonal to the direction of movement of the formation image (main scanning).
Shading correction circuit 20 is a circuit designed to correct the irregularities in the quantity of light which illuminate CCD line sensor 4. Correction data for the unevenness of exposure, determined at the time of factory shipment, is stored in advance in EEPROM 19. Using this correction data, the correction of output signals of CCD line sensor 4 is conducted.
Stepping motor 7 is controlled by stepping motor drive circuit 6 via I/O 9 according to the commands of CPU 13. Moreover, CCD line sensor 4 is controlled by CCD drive circuit 11 and image signals of CCD line sensor 4 for the main scanning direction are output to shading correction circuit 20. After irregularity correction by shading correction circuit 20, the output of shading correction circuit 20 is converted into digital data by A/D converter 10 and stored in memory 12.
Stepping motor drive circuit 6 intermittently moves carriage 2 at uniform intervals and, with each movement, the image output of CCD line sensor 4 is converted into digital data by A/D converter 10.
When the reading of film original 3 is completed, the data stored in memory 12 is output to host computer 15 via SCSI interface 14 and is used as the image data.
FIG. 2 is a flow chart which shows an operational process of a device as configured in FIG. 1. With this operational process, after the power source has been turned ON at step S30, an operational check of memory in the reader main body is performed at step S32 and initialization of the reading mechanical system is conducted at step S34. Image reading of film original 3 is performed at step S36 using the shading correction coefficient and the coefficient of the quantity of light ratio for the separated colors of the light source, wherein both coefficients are stored in advance (at the time of factory shipment) in EEPROM 19. In step S38, the image is transferred to host computer 15.
Since these devices require a memory element, such as EEPROM 19, higher costs are incurred. In particular, a costly process for the writing of EEPROM 19 at the time of factory shipment becomes necessary.
In addition, when the quantity of light of the white light source changes due to a change in voltage or a deterioration of the light source over time, correction data which is different from the correction data at the time of factory shipment becomes necessary. Therefore, the correction data stored in EEPROM 19 is no longer accurate and a corrected shading correction coefficient and a new measurement of the quantity of light ratio must be obtained. Obtaining this new data requires separation filter 17 to be mechanically switched each time a correction is performed. As a result, not only does the time required for the starting of the device become longer by the amount of the switching time of the separation filter, but a conspicuous wear of mechanical parts pertaining to insertion and removal of the separation filter is also incurred.